lsp: Store diagnostics for unloaded buffers (#13102)

To avoid loading buffers https://github.com/neovim/neovim/pull/12440
changed the logic to not process diagnostics for unloaded buffers.

This is problematic for language servers where compile errors or build
errors are reported via diagnostics. These errors may prevent the
language server from providing all functions and it is difficult for
users to debug it without having access to the errors.

For example, with eclipse.jdt.ls there may be a problem with gradle (the
build tool for java), it results in a diagnostics like this:

    org.gradle.toolingapi/build.gradle|1 col 1| Could not run build action using Gradle distribution 'https://services.gradle.org/distributions/gradle-4.8.1-bin.zip'.

This would be invisible to users unless the user happens to open the
right file. In this case the user would actually never see the error,
because the language server isn't attached to the build configuration
files.

This changes the behaviour to at least store the diagnostics. The other
operations which are more expensive are still skipped.

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